The instant invention will be described as it relates to adjusting an electrical characteristic of a cup core transformer by changing the dimensions of an air gap in the magnetic circuit of the transformer. However, the invention should not be so limited for other devices having air gap portions which, when altered will affect an electrical characteristic of the device (e.g., capacitors) would fall within the purview of the instant invention.
Ferrite core transformers are fabricated by forming a mixture of oxides of iron and zinc, together with manganese carbonate, which mixture is thereafter compacted or pressed, and then fixed to form a desired rigid geometric core configuration. The ferrite cores must initially be pressed into the desired shape with oversized dimensions, typically ranging from ten to twenty percent, so as to compensate for the subsequent shrinkage thereof during firing. Accordingly, it becomes readily apparent that whenever very close tolerances are required with respect to critical core dimensions, such as an air gap formed in sectioned cores, abrasive machine grinding or lapping operations have often been required heretofore.
Such auxiliary operations are both time consuming and expensive, and have been particularly required in connection with the manufacture of cup-shaped ferrite cores. The cup core transformer typically has a circular end wall connected to a thin cylindrical outer wall and an axially disposed tubular leg portion which defines an inner wall. The annular volume between the inner and outer walls accommodates at least one toroidal shaped coil. A planar circular cap is affixed to the open end of the transformer to enclose the device resulting in an air gap between the cap and the free end of the axially disposed tubular member. The cap has a central aperture therein which is aligned with the axial leg portion. Normally a magnetic slug is threadably inserted in the aperture for movement into the air gap towards the free end of the axial leg to alter the gap therebetween to adjust the reluctance of the magnetic circuit of the transformer and alter the inductance thereof.
Usually, the machining of the free end of the tubular leg is accomplished so as to insure that the air gap between the cap and the free end is greater than a predetermined value. The magnetic slug can then be threadably inserted through the aperture in the cap and extend into the air gap to decrease the effective distance therebetween. However, the use of such a magnetic slug is not only expensive, but the slug can become loose and undesirably alter the inductance of the magnetic circuit of the transformer when in use.
One technique which eliminates the use of the magnetic slug is described in copending U.S. patent application Ser. No. 761,084, filed Jan. 21, 1977, which is assigned to the instant assignee. In that application, the air gap between the cap and the free end of the axially disposed leg is sufficiently narrow so as to yield a reluctance less than desired. Adjustment to a higher reluctance value is accomplished by directing a laser beam through the aperture in the cap onto the leg to remove material therefrom to alter the gap and increase the reluctance of the magnetic circuit.
An alternative method is described in copending U.S. patent application Ser. No. 759,609, filed Jan. 14, 1977, which is also assigned to the instant assignee. In that application, material is removed from the outer or end walls of an inductive device, in a direction perpendicular to the flux path, to decrease the cross-sectional area and increase the reluctance of the transformer.
The above-referred to methods have been found to be most effective to increase the reluctance of the magnetic circuit of the transformer in order to decrease the inductance thereof. Both methods require that the air gap initially be smaller than desired to provide a reluctance less than desired. However, machining of the transformer is most difficult, in particular, where close tolerances are required. Thus, if careful control of the machining process is not exercised, the resultant air gap may be greater than required. Also, on occasion, it may be necessary to readjust a priorly adjusted device in which the air gap must be decreased. Such a decrease of the air gap cannot be accomplished using the above techniques.